This invention relates to novel solder compositions or solder creams, and is particularly concerned with the provision of solder compositions especially adapted for soldering components in densely packed microelectronic circuits and wherein the solder joints do not crack during thermal cycling of the system, e.g. computers, containing such circuits.
A solder cream is a material that is capable of being applied to a substrate or surface in a specific pattern using screening or analogous methods which can subsequently undergo fusing to provide an electrical joint or interface commonly described as a solder joint. The solder cream generally consists of metallic particles of various alloys such as tin-lead, tin-lead-silver, etc., contained in a vehicle including a flux such as wood rosin, or derivatives thereof.
Currently, microelectronic circuits are made by surface mounting chip carriers and other components on ceramic and plastic substrates by vapor phase soldering. Circuits similar to those used on printed circuit boards are produced on the substrates. In certain instances, the circuit is located on one side of the substrate, with pads being provided in certain areas, other parts of the circuit being covered with an insulating material.
The solder cream having a paste-like consistency is applied, for example by means of a silk screen, to the pads on the circuit board. Thereafter, the electronic components are carefully positioned with their peripheral contacts on the solder cream-coated pads. When all of the components are thus in place, the board, with such components temporarily positioned and retained thereon by the solder cream, can be placed in a vapor reflow system and subjected to a sufficiently high temperature to cause the metal content of the solder cream to liquefy and the contacts of the electronic components to be fused and to adhere to the pads on the circuit board. Examples of solder compositions are described in U.S. Pat. No. 3,684,533 to Conwicke and U.S. Pat. Nos. 4,373,974, 4,509,994 and 4,531,986, to Barajas.
However, densely packed microelectronic circuits often fail prematurely because the solder joints employed therein crack during the thermal cycling associated with normal operation of modern fast computers containing such circuits.
It is accordingly an object of the present invention to provide an improved solder composition or solder cream which is especially adapted for soldering components in electronic circuitry, particularly densely packed microelectronic circuits.
Another object is the provision of a solder composition of the above type employing solder alloys which avoids cracking of solder joints in microelectronic circuits, particularly those employed in modern fast computers, as result of thermal cycling associated with normal operation thereof